1. Field of Invention
The invention relates to a transmission rate adjusting method in a wireless local area network (WLAN) and, in particular, to a cross-layer rate adaptation mechanism for WLAN in a communication system with multiple sets of transmission rates, which selects one transmission rate from several transmission rates in the physical layer (PHY) through a medium access control (MAC) layer.
2. Related Art
Wireless communications utilize radio waves to achieve signal transmissions. It has developed for long time and has been used in widely many aspects, including military purposes, navigations, aviation, and scientific researches. Using wireless communications in public communication services is very recent. The introduction of the honeycomb mobile communication technology has further facilitated the applications of wireless communications in daily life.
In the past few years, wireless communications have made tremendous progress. Its popularity has been increased by a factor that is far larger than the accumulated usage in last few decades. The basic reasons are that in addition to technological advances, the trend of commercialized global communications results in a free, competitive market and cheap, good products.
Integrating wireless technology with networks is also a mainstream in the modern development. According to their covered transmission ranges, the current wireless network can be categorized into four types: the wireless wide area network (WWAN), the wireless metropolitan area network (WMAN), the wireless local area network (WLAN), and the wireless personal area network (WPAN)
The wireless communications have brought the convenience in our lives, and also made the wireless local area networks almost as popular as the conventional wired Internet (e.g. the Ethernet). Therefore, many scholars have devoted to the study for the field. The main function of the wireless local area network is to provide high data rate transmissions. The transmission rate has been as fast as 54 Mbps. Using the wireless network to access the Internet enables people to enjoy the convenience. Therefore, they are particular suitable for uses at homes, conference rooms, offices, airports, and restaurants.
The standard for wireless local area networks has been evolved from the early IEEE 802.11b to the current IEEE 802.11a/g. In a wireless environment, how to adopt the most suitable modulation technology for data transmissions according to the state variation of the channels is the most important issue in the field. In the past, there were a lot of related techniques for adapting transmission rate. They can be roughly classified into three types according to their properties. The first type includes the methods proposed, for example, by Lucent and AMD, which is designed according to whether a transmission is successful without worrying about the channel links. The most famous technique is the AutoRate Fallback (ARF) mechanism proposed by Bell Lab in 1997 and adopted in the WaveLAN-II network card produced by Lucent. WaveLAN-II adopts the IEEE 802.11 standard, supporting both 1 Mbps and 2 Mbps transmission rates. The ARF mechanism switches between the two transmission rates (1 Mbps and 2 Mbps) according to the value of a counter, which counts the numbers of successful ACK frame or failure times. If the transmitting end does not receive correct ACK frames in consecutive two times, the system restarts the counter and lowers the rate for the next transmission. When the transmitting end successfully receives the ACK frame for tens times, the system raises the transmission rate and restarts the counter. Obviously, the ARF mechanism does not have a sufficiently fast reaction time to adjust the transmission rate according to the variation of the wireless channel. Moreover, the transmission rate adaptation method proposed by AMD, the Transmission Statistics, also uses whether a transmission is successful as the criterion. The second type includes the Rate-dependent Packet Shaping method proposed by AT&T. The transmission rate varies with the length of the transmitted data. Its purpose is to restrict that each transmission time is the same, and it achieves fairness for the use of the wireless channels. The third type is designed according to the link quality; different transmission rate thresholds are established according to the state of the wireless channel. Most of these techniques are proposed by Philips. For example, the state of link quality can be obtained from several factors such as the received signal strength (RSS), the noise margin, or the path loss. The system evaluates the quality of the previous link and then adjusts the transmission rate according to the thresholds of different transmission rate. However, the operation of the above-mentioned mechanisms depends on a basic assumption that a signal is successfully transmitted. If a transmission fails, the system loses the information of the link quality and cannot make any rate adjustment.